Multiple minor changes

Hi,

I made some changes that may be of interest. The following things have been adjusted:

* introduced new `getHelpText()` method and used it in `showHelp()` method to give people the freedom to enhance the help text before displaying them
  * made padding for help text configurable (default is still 25)
  * suppressed PHP warning when description is not given in `showHelp()` method (added test for that)
* used multibyte safe PHP string methods
* made private methods protected as that should be sufficient as visibilty
* added `phpunit.xml` file with autoloading and warnings as exceptions (to get failing tests on  PHP warnings) etc.
* added cheap `Makefile` with some targets:
  * test: runs phpunit tests
  * install-composer: installs composer
  * install-dependencies: installs/updates all vendor libraries using composer
  * install-dev-dependencies: installs/updates all vendor libraries necessary for development
* added `require-dev` section in `composer.json` with phpunit requirement
* some whitespace changes (there were some mixed tabs and spaces in files)
* added `.gitignore` file for vendor and build directories (composer.lock file as well as getopt-php is a library)
* adjusted README:
  * included `getOptions()` method
  * added development section with information about `make` and `composer`

I'm not sure whether the multibyte safe method usage is necessary and am open to feedback. :-)

Hope you like it.

Kind regards,

Steffen Gransow